The aim of this study was to characterize the antioxidant properties of three ascorbic acid (AsA) derivatives O-substituted at the C-2 position of AsA: ascorbic acid 2-glucoside (AA-2G), ascorbic acid 2-phosphate (AA-2P), and ascorbic acid 2-sulfate (AA-2S). The radical-scavenging activities of these AsA derivatives and some common low molecular-weight antioxidants such as uric acid and reduced glutathione (GSH) against 1,1-diphenyl-picrylhydrazyl (DPPH) radical, 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation (ABTS^<・+>) or galvinoxyl radical were kinetically and stoichiometrically evaluated under pH-controlled conditions. AA-2G, AA-2P and AA-2S slowly and continuously reacted with DPPH radical and ABTS^<・+>, but not with galvinoxyl radical. They effectively reacted with DPPH radical under acidic conditions and with ABTS^<・+> under neutral conditions. In contrast, AsA immediately quenched all species of radicals tested at all pH values. The reactivity of Trolox, a water-soluble vitamin E analogue, was comparable to that of AsA in terms of kinetics and stoichiometrics. Uric acid and GSH exhibited long-lasting radical-scavenging activity against these radicals under certain pH conditions. The radical-scavenging profiles of AA-2G, AA-2P and AA-2S were closer to those of uric acid and GSH rather than to that of AsA. The number of radicals scavenged by one molecule of the AsA derivatives was equal to or greater than that by AsA or Trolox under the appropriate conditions. Furthermore, inhibitory effects of AA-2G, AA-2P and AA-2S on 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH)-induced oxidative hemolysis of sheep erythrocytes were studied and were compared with those of AsA and other antioxidants. The order of the inhibition efficiency was AA-2S&ge;Trolox=uric acid&ge;AA-2P>AA-2G=AsA>GSH. Although the reactivity of the AsA derivatives against AAPH-derived peroxyl radical (ROO^・) was much lower than that of AsA, the derivatives exerted equal or more potent protective effects on AAPH-induced hemolysis. In addition, the AsA derivatives were found to react per se with DPPH radical and ABTS^<・+> and ROO^・, not via AsA as an intermediate. These results suggest that the AsA derivatives per se act as biologically effective antioxidants under moderate oxidative stress and that AA-2G and AA-2P may be able to act under severe oxidative stress after enzymatic conversion to AsA in vivo.